SCD4x Breaking the size barrier in CO

www.sensirion.com Version 1 – January 2021 1/22

High accuracy: ±(40 ppm + 5 %)

Digital interface I2C with digital output signal

Integrated temperature and humidity sensor

Adjustable current-consumption down to

< 0.4 mA avg. @ 5 V, 1 meas. / 5 minutes

SCD4x Breaking the size barrier in CO2 sensing

Products Details

SCD40-D-R2 Base accuracy, specified range 400 – 2’000 ppm

SCD41-D-R2 High accuracy, specified range 400 – 5’000 ppm, single shot mode supported

Full product list on page 21

Functional Block Diagram Device Overview

Product Summary

The SCD4x is Sensirion’s next generation miniature CO2 sensor. This sensor builds on the photoacoustic sensing principle and Sensirion’s patented PAsens® and CMOSens® technology to offer high accuracy at an unmatched price and smallest form factor. SMD assembly allows cost- and space-effective integration of the sensor combined with maximal freedom of design. On-chip signal compensation is realized with the build-in SHT4x humidity

and temperature sensor.

CO2 is a key indicator for indoor air quality as high levels compromise humans’ cognitive performance and well-being. The SCD4x enables smart ventilation systems to regulate ventilation in the most energy-efficient and human-friendly way. Moreover, indoor air quality monitors and other connected devices based on the SCD4x can help maintaining low CO2 concentration for a healthy, productive environment.

Features

Photoacoustic sensor technology PASens®

Smallest form factor: 10.1 x 10.1 x 6.5 mm3

Surface-mount device for effective assembly

Large output range: 0 ppm – 40’000 ppm

Large supply voltage range: 2.4 – 5.5 V


Table of Contents 1 Sensor Performance ......................................................................................................................................... 3

1.1 CO2 Sensing Performance .......................................................................................................................... 3 1.2 Humidity Sensing Performance ................................................................................................................... 3

1.3 Temperature Sensing Performance5 ........................................................................................................... 3 2 Specifications .................................................................................................................................................... 4

2.1 Electrical Specifications ............................................................................................................................... 4 2.2 Absolute Maximum Ratings ......................................................................................................................... 4 2.3 Interface Specifications ............................................................................................................................... 5 2.4 Timing Specifications................................................................................................................................... 6

2.5 Material Contents ........................................................................................................................................ 6 3 Digital Interface Description ............................................................................................................................. 7

3.1 Power-Up and Communication Start ........................................................................................................... 7 3.2 Data type & length ....................................................................................................................................... 7 3.3 Command Sequence Types ........................................................................................................................ 7 3.4 SCD4x Command Overview ........................................................................................................................ 8 3.5 Basic Commands ........................................................................................................................................ 9 3.6 On-Chip Output Signal Compensation ...................................................................................................... 10

3.7 Field Calibration ........................................................................................................................................ 12 3.8 Low Power operation ................................................................................................................................. 14 3.9 Advanced Features ................................................................................................................................... 15 3.10 Low power single shot (SCD41) ................................................................................................................ 17 3.11 Checksum Calculation ............................................................................................................................... 18

4 Mechanical specifications .............................................................................................................................. 19

4.1 Package Outline ........................................................................................................................................ 19 4.2 Land Pattern .............................................................................................................................................. 19 4.3 Tape & Reel Package................................................................................................................................ 20

4.4 Moisture Sensitivity Level .......................................................................................................................... 20 4.5 Soldering Instructions ................................................................................................................................ 21

4.6 Traceability ................................................................................................................................................ 21 5 Ordering Information ....................................................................................................................................... 21 6 Revision History .............................................................................................................................................. 21


1 Sensor Performance

1.1 CO2 Sensing Performance

Default conditions of 25 °C, 50 % RH, 1013 mbar, periodic measurement (i.e. high performance mode) and 3.3 V supply voltage apply to values in the table below, unless otherwise stated.

Parameter Conditions Value

CO2 output range1 - 0 – 40’000 ppm

SCD40 CO2 measurement accuracy2 400 ppm – 2’000 ppm ± (50 ppm + 5% of reading)

SCD41 CO2 measurement accuracy2 400 ppm – 5’000 ppm ± (40 ppm + 5% of reading)

Repeatability Typical ± 10 ppm

Response time3 τ63%, typical 60 s

Accuracy drift per year with automatic self-calibration algorithm enabled4

Typical ± (5 ppm + 0.5 % of reading)

Table 1: SCD40 and SCD41 CO2 sensor specifications

1.2 Humidity Sensing Performance5


Humidity measurement range - 0 %RH – 100 %RH

Accuracy (typ.) 15 °C – 35 °C, 20 %RH – 65 %RH 6 % RH

-10 °C – 60 °C, 0 %RH – 100 %RH 9 % RH

Repeatability Typical ± 0.4 %RH


Accuracy drift - < 0.25 %RH / year

Table 2: SCD4x humidity sensor specifications

1.3 Temperature Sensing Performance5


Temperature measurement range - - 10°C – 60°C

Accuracy (typ.) 15 °C – 35 °C 0.8 °C

-10 °C – 60 °C 1.5 °C

Repeatability - ± 0.1°C


Accuracy drift - < 0.03 °C / year

Table 3: SCD4x temperature sensor specifications

1 Exposure to CO2 concentrations smaller than 400 ppm can affect the accuracy of the sensor if the automatic self-calibration (ASC) is on. 2 Deviation to a high-precision reference. Accuracy is fulfilled by > 90% of the sensors after calibration. Rough handling, shipping and soldering reduces the accuracy of the sensor. Accuracy is restored with FRC or ASC recalibration features. Accuracy is based on tests with gas mixtures having a tolerance of ± 1.5%. 3 Time for achieving 63% of a respective step function when operating the SCD41 Evaluation Kit with default measurement mode. Response time depends on design-in, signal update rate and environment of the sensor in the final application. 4 For proper function of ASC field-calibration algorithm SCD4x has to be exposed to air with CO2 concentration 400 ppm regularly. Maximum accuracy drift per year estimated from stress tests is ± (5 ppm + 2 % of reading). Higher drift values may occur if the sensor is not handled according to its handling instructions. 5 Design-in of the SCD4x in final application, self-heating of the senor and the environment impacts the accuracy of the RH/T sensor. To realize indicated specifications, the temperature-offset of the SCD4x inside the customer device must be set correctly (see chapter 3.6). Best RH/T accuracy is realized when operating the SCD4x in low power continuous measurement mode.


2 Specifications

2.1 Electrical Specifications

Parameter Symbol Conditions Min. Typical Max. Units

Supply voltage DC VDD 2.4 5.5 V

Voltage ripple peak to peak VRPP 30 mV

Peak supply current6 VDD = 3.3 V 175 205 mA

VDD = 5 V 115 137 mA

Average supply current periodic measurement (default mode)

IDD VDD = 3.3 V 15 18 mA

VDD = 5 V 11 13 mA

Average supply current low power mode IDD VDD = 3.3 V 3.2 3.5 mA

VDD = 5 V 2.8 3 mA

Average supply current single shot, 1 measurement / 5 minutes (SCD41 only)7

IDD VDD = 3.3 V 0.45 0.5 mA

VDD = 5 V 0.36 0.4 mA

Input high level voltage VIH 0.7 x VDD 1 x VDD -

Input low level voltage VIL 0.3 x VDD -

Output low level voltage VOL 3 mA sink current 0.66 V

Table 4 SCD4x electrical specifications

2.2 Absolute Maximum Ratings

Stress levels beyond those listed in Table 5 may cause permanent damage to the device. Exposure to minimum/maximum rating conditions for extended periods may affect sensor performance and reliability of the device.


Temperature operating conditions -10 – 60°C

Humidity operating conditions8 Non-condensing. 0 – 95 %RH

MSL Level 3

DC supply voltage - 0.3 V – 6.0 V

Max voltage on pins SDA, SCL, GND – 0.3 V to VDD+0.3 V

Input current on pins SDA, SCL, GND +- 100 mA

Short term storage temperature9 - 40°C – 70°C

Recommended storage temperature 10 °C – 50 °C

ESD HBM 2 kV

ESD CDM 500 V

Maintenance Interval Maintenance free when ASC field-calibration algorithm10 is used.

None

Sensor lifetime11 Typical operating conditions > 10 years

Table 5: SCD4x operation conditions, lifetime and maximum ratings

6 Power supply should be designed with respect to peak current. 7 On-demand measurement with freely adjustable interval. See chapter 3.10 8 Accuracy can be reduced at relative humidity levels lower than 10 %. 9 Short term storage refers to temporary conditions during e.g. transport. 10 For proper function of ASC field-calibration algorithm the SCD4x has to be exposed to outdoor air with 400 ppm concentration regularly. 11 Sensor tested over simulated lifetime of > 10 years for indoor environment mission profile


2.3 Interface Specifications

The SCD4x comes in an LGA package (Table 6). The package outline is schematically displayed in chapter 4.1. The landing pattern of the SCD4x can be found in chapter 4.2.

Name Comments

VDD Supply voltage

VDDH Supply voltage IR source, must be connected to VDD on customer PCB

GND Ground contact

SDA I2C Serial data, bidirectional

SCL I2C Serial clock

DNC Do not connect, pads must be soldered to a floating pad on the customer PCB

Table 6 Pin assignment (top view). The notched corner of the protection membrane serves as a polarity mark to indicate pin 1 location.

VDD and VDDH are used to supply the sensor and must always be kept at the same voltage, i.e. should both be connected to the same power supply. The combined maximum current drawn on VDD and VDDH is indicated in Table 4. Care should be taken to choose a low noise power supply (preferably a low-dropout regulator, LDO with output ripple of less than 30 mV p-p), which is adequately dimensioned for the relatively large peak currents. Power supply configurations with large transient voltage drops are to be avoided to ensure proper sensor operation. SCL is used to synchronize the I2C communication between the master (microcontroller) and the slave (sensor). The SDA pin is used to transfer data to and from the sensor. For safe communication, the timing specifications defined in the I2C manual12 must be met. Both SCL and SDA lines should be connected to external pull-up resistors (e.g. Rp = 10 kΩ, see Figure 1). To avoid signal contention, the microcontroller must only drive SDA and SCL low. For dimensioning resistor sizes please take bus capacity and communication frequency into account (see example in Section 7.1 of NXPs I2C Manual for more details10). It should be noted that pull-up resistors may be included in I/O circuits of microcontrollers.

Figure 1: Typical application circuit (for better clarity in the image, the positioning of the pins does not reflect the positions on the real sensor). VDD and VDDH must be connected to each other close to the sensor on the customer PCB.

12 http://www.nxp.com/documents/user_manual/UM10204.pdf

http://www.nxp.com/documents/user_manual/UM10204.pdf


2.4 Timing Specifications

Table 7 list the timings of the ASIC part and does not reflect the availability or usefulness of the sensor readings. The SCD4x supports the I2C “standard-mode” as is described elsewhere (see footnote 12).

Parameter Condition Min. Max. Unit

Power-up time After hard reset, VDD ≥ 2.25 V - 1000 ms

Soft reset time After re-initialization (i.e. reinit) - 1000 ms

SCL clock frequency - 0 100 kHz

Table 7 System timing specifications.

2.5 Material Contents

The device is fully RoHS compliant.


3 Digital Interface Description All SCD4x commands and data are mapped to a 16-bit address space.

SCD4x Hex. Code

I2C address 0x62

Table 8 I2C device address.

3.1 Power-Up and Communication Start

The sensor starts powering-up after reaching the power-up threshold voltage VDD,Min = 2.25 V. After reaching this threshold voltage, the sensor needs 1000 ms to enter the idle state. Once the idle state is entered it is ready to receive commands from the master. Each transmission sequence begins with a START condition (S) and ends with a STOP condition (P) as described in the I2C-bus specification.

3.2 Data type & length

Data sent to and received from the sensor consists of a sequence of 16-bit words (each to be interpreted as unsigned integer, most significant byte transmitted first). Each word is immediately succeeded by an 8-bit CRC. In write direction it is mandatory to transmit the checksum. In read direction it is up to the master to decide if it wants to process the checksum (see chapter 3.11).

3.3 Command Sequence Types

The SCD4x features three different I2C command sequence types: read I2C sequences, write I2C sequences and send I2C command sequences. Figure 2 illustrates how the I2C communication for the different sequence types is built-up.

Figure 2: Command Sequence types: write sequence, send command sequence and read sequence.

After issuing read sequence commands and sending the ACK Bit the sensor needs the execution time (see Table 9) to respond to the I2C read header with an ACK bit. Hence, it is required to wait the command execution time before issuing the read header. Commands must not be sent while a previous command is being processed.


3.4 SCD4x Command Overview

Table 9: List of SCD4x sensor commands. Detailed description of SCD4x commands can be found further down.

Domain Command Hex. Code I2C sequence type

(see chapter 3.3)

Execution time [ms]

Basic Commands

Chapter 3.5

start_periodic_measurement 0x21b1 send command -

read_measurement 0xec05 read 1

stop_periodic_measurement 0x3f86 send command 500

On-chip output signal compensation

Chapter 3.6

set_temperature_offset 0x241d write 1

get_temperature_offset 0x2318 read 1

set_sensor_alitutude 0x2427 write 1

get_sensor_alitutude 0x2322 read 1

set_ambient_pressure 0xe000 write 1

Field calibration

Chapter 3.7

perform_forced_recalibration 0x362f write and read 400

set_automatic_self_calibration_enabled 0x2416 write 1

get_automatic_self_calibration_enabled 0x2313 read 1

Low power

Chapter 3.8

start_low_power_periodic_measurement 0x21ac send command -

get_data_ready_status 0xe4b8 read 1

Advanced features

Chapter 3.9

persist_settings 0x3615 send command 800

get_serial_number 0x3682 read 1

perform_self_test 0x3639 read 5500

perform_factory_reset 0x3632 send command 1200

reinit 0x3646 send command 20

Low power single shot (SCD41 only) Chapter 3.10

measure_single_shot 0x219d send command 1350

measure_single_shot_rht_only 0x2196 send command 50


3.5 Basic Commands

This section lists the basic SCD4x commands that are necessary to start a periodic measurement and subsequently read out the sensor outputs. The typical communication sequence between the I2C master (e.g., a microcontroller) and the SCD4x sensor is as follows:

1. The sensor is powered up 2. The I2C master sends a start periodic measurement command. Signal update interval is 5 seconds. 3. The I2C master periodically reads out data with the read measurement sequence. 4. To put the sensor back to idle mode, the I2C master sends a stop periodic measurement command.

While a periodic measurement is running, no other commands must be issued with the exception of read measurement, get data ready status, stop periodic measurement and set ambient pressure.

3.5.1 start_periodic_measurement

Description: start periodic measurement, signal update interval is 5 seconds. The default periodic measurement mode is also referred to as ‘high performance mode’.

Table 10: start_periodic_measurement I2C sequence description

Write (hexadecimal)

Input parameter: - Response parameter: - Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x21b1 - - - - not applicable

Example: start periodic measurement

Write 0x21b1

(hexadecimal) Command

3.5.2 read_measurement

Description: read sensor output. The measurement data can only be read out once per signal update interval as the buffer is emptied upon read-out. If no data is available in the buffer, the sensor returns a NACK. To avoid a NACK response the get_data_ready_status can be issued to check data status (see chapter 3.8.2 for further details).The I2C master can abort the read transfer with a NACK followed by a STOP condition after any data byte if the user is not interested in subsequent data.

Table 11: read_measurment I2C sequence description

Write (hexadecimal)

Input parameter: - Response parameter: CO2, Temperature, Relative Humidity

Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0xec05 - -

3 CO2 [ppm] = word[0]

1 3 T = - 45 + 175 * word[1] / 216

3 RH = 100 * word[2] / 216

Example: read sensor output (500 ppm, 25 °C, 37 % RH)

Write 0xec05


Wait 1 ms command execution time

Response 0x01f4 0x7b 0x6667 0xa2 0x5eb9 0x3c

(hexadecimal) CO2 = 500 ppm CRC of 0x01f4 Temp. = 25 °C CRC of 0x6667 RH = 37 % CRC of 0x5eb9


3.5.3 stop_periodic_measurement

Description: stop periodic measurement for sensor configuration or to save power.

Table 12: stop_periodic_measurement I2C sequence description

Write (hexadecimal)


0x3f86 - - - - 500

Example: stop periodic measurement

Write 0x3f86


3.6 On-Chip Output Signal Compensation

The SCD4x features on-chip signal compensation to counteract pressure and temperature effects. Feeding the SCD4x with the pressure or altitude enables highest accuracy of the CO2 output signal across a large pressure range. Setting the temperature offset improves the accuracy of the relative humidity and temperature output signal. Note that the temperature offset does not impact the accuracy of the CO2 output. To change or read sensor settings, the SCD4x must be in idle mode. A typical sequence between the I2C master and the SCD4x is described as follows:

1. If the sensor is operated in a periodic measurement mode, the I2C master sends a stop_periodic_measurement command

2. The I2C master sends one or several commands to get or set the sensor settings 3. If configurations shall be preserved after power-cycle events, the persist_settings command is sent (see chapter 3.9.1) 4. The I2C master sends a start measurement command to set the sensor in the operating mode again

3.6.1 set_temperature_offset

Description: The temperature offset has no influence on the SCD4x CO2 accuracy. Setting the temperature offset of the SCD4x inside the customer device correctly allows the user to leverage the RH and T output signal. Note that the temperature offset can depend on various factors such as the SCD4x measurement mode, self-heating of close components, the ambient temperature and air flow. Thus, the SCD4x temperature offset should be determined inside the customer device under its typical operation and in thermal equilibrium. Per default, the temperature offset is set to 4° C. To save the setting to the EEPROM, the persist setting (see chapter 3.9.1) command must be issued. Equation (1) shows how the characteristic temperature offset can be obtained.

𝑇𝑜𝑓𝑓𝑠𝑒𝑡_𝑎𝑐𝑡𝑢𝑎𝑙 = 𝑇𝑆𝐶𝐷40 − 𝑇𝑅𝑒𝑓𝑒𝑟𝑒𝑛𝑐𝑒 + 𝑇𝑜𝑓𝑓𝑠𝑒𝑡_ 𝑝𝑟𝑒𝑣𝑖𝑜𝑢𝑠 (1)

Table 13: set_temperature_offset I2C sequence description

Write (hexadecimal)

Input parameter: Offset temperature Response parameter: - Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x241d 3 Toffset [°C] = 175 * word[0] / 216 - - 1

Example: set temperature offset to 5.4 °C

Write 0x241d 0x07e6 0x48

(hexadecimal) Command Toffset = 5.4 °C CRC of 0x7e6


3.6.2 get_temperature_offset

Table 14: get_temperature_offset I2C sequence description

Write (hexadecimal)

Input parameter: - Response parameter: Offset temperature Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x2318 - - 3 Toffset [°C] = 175 * word[0] / 216 1

Example: temperature offset is 6.2 °C

Write 0x2318



Response 0x0912 0x63

(hexadecimal) Toffset = 6.2 °C CRC of 0x0912

3.6.3 set_sensor_altitude

Description: Reading and writing of the sensor altitude must be done while the SCD4x is in idle mode. Typically, the sensor

altitude is set once after device installation. To save the setting to the EEPROM, the persist setting (see chapter 3.9.1) command

must be issued. Per default, the sensor altitude is set to 0 meter above sea-level.

Table 15: set_sensor_altitude I2C sequence description

Write (hexadecimal)

Input parameter: Sensor altitude Response parameter: - Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x2427 3 Sensor altitude [m] = word[0] - - 1

Example: set sensor altitude to 1’950 m.a.s.l.

Write 0x2427 0x079e 0x09

(hexadecimal) Command Sensor altitude = 1’950 m CRC of 0x79e

3.6.4 get_sensor_altitude

Table 16: get_sensor_altitude I2C sequence description

Write (hexadecimal)

Input parameter: - Response parameter: Sensor altitude Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x2322 - - 3 Sensor altitude [m] = word[0] 1

Example: sensor altitude is 1’100 m.a.s.l.

Write 0x2322



Response 0x044c 0x42

(hexadecimal) Sensor altitude = 1’100 m CRC of 0x044c


3.6.5 set_ambient_pressure

Description: The set_ambient_pressure command can be sent during periodic measurements to enable continuous pressure compensation. Note that setting an ambient pressure to the sensor using set_ambient_pressure overrides any pressure compensation based on a previously set sensor altitude.

Table 17: set_ambient_pressure I2C sequence description

Write (hexadecimal)

Input parameter: Ambient pressure Response parameter: - Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0xe000 3 Ambient P [Pa] = 100* word[0]

- - 1

Example: set ambient pressure to 98’700 Pa

Write 0xe000 0x03db 0x42

(hexadecimal) Command Ambient P = 98’700 Pa CRC of 0x03db

3.7 Field Calibration

To realize high initial and long-term accuracy, the SCD4x includes two field calibration features. Forced recalibration (FRC) enables restoring highest accuracy with the assistance of a CO2 reference value immediately. Typically, FRC is applied to compensate for drifts originating from the sensor assembly process or other extensive stresses. Automatic self-calibration (ASC) ensures highest long-term stability of the SCD4x without the need of manual action steps from the user. The automatic self-calibration algorithm is based on the assumption that the sensor is exposed to the atmospheric CO2 concentration of 400 ppm at least once per week.

3.7.1 perform_forced_recalibration

Description: To successfully conduct an accurate forced recalibration, the following steps need to be carried out: 1. Operate the SCD4x in a periodic measurement mode for > 3 minutes in an environment with homogenous and constant

CO2 concentration. 2. Stop periodic measurement. Wait 500 ms. 3. Subsequently issue the perform_forced_recalibration command and optionally read out the FRC correction (i.e. the

magnitude of the correction).

A return value of 0xffff indicates that the forced recalibration failed Note that the sensor will fail to perform a forced recalibration if it was not operated before sending the command. Please make sure that the sensor is operated at the voltage desired for the application when applying the forced recalibration sequence.


Table 18: perform_forced_recalibration I2C sequence description

Write (hexadecimal)

Input parameter: Target CO2 concentration Response parameter: FRC-correction Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x362f 3 Target concentration [ppm CO2] = word[0]

3 FRC correction [ppm CO2] = word[0] – 0x8000

word[0] = 0xfff in case of failed FRC

400

Example: perform forced recalibration, reference CO2 concentration is 490 ppm

Write 0x362f 0x01e0 0xb4

(hexadecimal) Command Input: 490 ppm CRC of 0x01e0


Response 0x7fce 0x7b

(hexadecimal) Response: - 50 ppm CRC of 0x7fce

3.7.2 set_automatic_self_calibration_enabled

Description: Set the current state (enabled / disabled) of the automatic self-calibration. By default, ASC is enabled. To save the setting to the EEPROM, the persist setting (see chapter 3.9.1) command must be issued.

Table 19: set_automatic_self_calibration_enabled I2C sequence description.

Write (hexadecimal)

Input parameter: ASC enabled Response parameter: - Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x2416 3 word[0] = 1 → ASC enabled

word[0] = 0 → ASC not enabled

- - 1

Example: set automatic self-calibration status: enabled

Write 0x2416 0x0001 0xB0

(hexadecimal) Command ASC enabled CRC of 0x0001

3.7.3 get_automatic_self_calibration_enabled

Table 20: get_automatic_self_calibration_enabled I2C sequence description

Write (hexadecimal)

Input parameter: - Response parameter: ASC enabled Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x2313 - - 3 word[0] = 1 → ASC enabled

word[0] = 0 → ASC disabled

1

Example: read automatic self-calibration status: disabled

Write 0x2313




(hexadecimal) ASC disabled CRC of 0x0000


3.8 Low Power operation

To enable use-cases with a constrained power-budget, the SCD4x features a low power continuous measurement mode with signal update interval of 30 seconds. While the low power mode saves power and reduces self-heating of the sensor, the low power continuous measurement mode has a longer response time. The low power continuous measurement mode is initiated and read-out in a similar manner as the default periodic measurement. Please consult chapter 3.5.2 for further instructions. To avoid NACK when reading the SCD4x output signals, the get_data_ready_status command can be used to check whether new measurement data is available for read-out.

3.8.1 start_low_power_periodic_measurement

Description: start low power periodic measurement, signal update interval is 30 seconds.

Table 21: start_low_power_periodic_measurement I2C sequence description

Write (hexadecimal)


0x21ac - - - - not applicable

Example: start low power periodic measurement

Write 0x21ac


3.8.2 get_data_ready_status

Table 22: get_data_ready_status I2C sequence description

Write (hexadecimal)

Input parameter: - Response parameter: data ready status Max. command duration [ms] length [bytes]

signal conversion

length [bytes] signal conversion

0xe4b8 - - 3 If ‘last 11 bits are 0’ → data not ready

else → data ready for read-out

1

Example: read data ready status: data not ready

Write 0xe4b8



Response 0x8000 0xa2

(hexadecimal) Last 11 bits are 0 → data not ready CRC of 0x8000


3.9 Advanced Features

3.9.1 persist_settings

Description: Configuration settings such as the temperature offset, sensor altitude and the ASC enabled/disabled parameter are by default stored in the volatile memory (RAM) only and will be lost after a power-cycle or when using the power_down command. The persist_settings command stores the current configuration in the EEPROM of the SCD4x, making them persistent across power-cycling. To avoid unnecessary wear of the EEPROM, the persist_settings command should only be sent when persistence is required and if actual changes to the configuration have been made (the EEPROM is guaranteed to endure at least 2000 write cycles before failure). Note that field calibration history (i.e. FRC and ASC, see chapter 3.7) is automatically stored in a separate EEPROM dimensioned for the specified sensor lifetime.

Table 23: persist_settings I2C sequence description

Write (hexadecimal)


0x3615 - - - - 800

Example: persist settings

Write 0x3615


3.9.2 get_serial_number

Description: Reading out the serial number can be used to identify the chip and to verify the presence of the sensor. The get serial number command returns 3 words, and every word is followed by an 8-bit CRC checksum. Together, the 3 words constitute a unique serial number with a length of 48 bits (big endian format).

Table 24: get_serial_number I2C sequence description

Write (hexadecimal)

Input parameter: - Response parameter: serial number Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x3682 - - 9 Serial number = word[0] << 32 | word[1] << 16 | word[2]

1

Example: serial number is 273’325’796’834’238

Write 0x3682



Response 0xf896 0x31 0x9f07 0xc2 0x3bbe 0x89

(hexadecimal) word[0] CRC of 0xf896 word[1] CRC of 0x9f07 word[2] CRC of 0x3bbe


3.9.3 perform_self_test

Description: The perform_self_test feature can be used as an end-of-line test to confirm sensor functionality and the customer power supply to the sensor.

Table 25: perform_self_test I2C sequence description

Write (hexadecimal)

Input parameter: - Response parameter: sensor status Max. command duration [ms] length [bytes] signal conversion length [bytes] signal conversion

0x3639 - - 3 word[0] = 0 → no malfunction detected

word[0] ≠ 0 → malfunction detected

5500

Example: perform self-test, no malfunction detected

Write 0x3639




(hexadecimal) No malfunction detected CRC of 0x0000

3.9.4 perfom_factory_reset

Description: The perform_factory_reset command resets all configuration settings stored in the EEPROM and erases the FRC and ASC algorithm history.

Table 26: perform_factory_reset I2C sequence description

Write (hexadecimal)


0x3632 - - - - 1200

Example: perform factory reset

Write 0x3632


3.9.5 reinit

Description: The reinit command reinitializes the sensor by reloading user settings from EEPROM. Before sending the reinit command, the stop measurement command must be issued. If reinit command does not trigger the desired re-initialization, a power-cycle should be applied to the SCD4x.

Table 27: reinit I2C sequence description

Write (hexadecimal)


0x3646 - - - - 20

Example: reinit

Write 0x3646

(hexadecimal) Command re-initialization


3.10 Low power single shot (SCD41)

In addition to continuous measurement modes, the SCD41 features single shot measurement (i.e. on-demand measurement). The typical communication sequence between the I2C master and the SCD41 sensor is as follows:

1. The sensor is powered up 2. The I2C master sends a single shot command and waits for the indicated max. command duration time 3. The I2C master reads out data with the read measurement sequence (chapter 3.5.2).

To reduce noise-levels, the I2C master can perform several single shot measurements and average of the CO2 output values. Same as with the continuous measurement modes, the automatic self-calibration (ASC) is enabled per default. The automatic self-calibration is optimized for a measurement interval of 5 minutes. Longer measurement interval will result in less frequent self-calibration sequences. Please consult Chapter 3.7 for further description on the automatic-self calibration and the corresponding commands.

3.10.1 measure_single_shot

Description: On-demand measurement of CO2 concentration, relative humidity and temperature. The sensor output is read with the read_measurement command (chapter 3.5.2).

Table 28: measure_single_shot I2C sequence description

Write (hexadecimal)


0x219d - - - - 1350

Example: measure single shot

Write 0x219d


3.10.2 measure_single_shot_rht_only

Description: On-demand measurement of relative humidity and temperature only. The sensor output is read with the read_measurement command (chapter 3.5.2). CO2 output is returned as 0 ppm.

Table 29: measure_single_shot_rht_only I2C sequence description

Write (hexadecimal)


0x2196 - - - - 50

Example: measure single shot, RH and T output only

Write 0x2196



3.11 Checksum Calculation

The 8-bit CRC checksum transmitted after each data word is generated by a CRC algorithm. Its properties are displayed in Table 30. The CRC covers the contents of the two previously transmitted data bytes. To calculate the checksum only these two previously transmitted data bytes are used. Note that command words are not followed by CRC.

Property Value Example code (C/C++)

Name CRC-8 #define CRC8_POLYNOMIAL 0x31 #define CRC8_INIT 0xFF uint8_t sensirion_common_generate_crc(const uint8_t* data, uint16_t count) { uint16_t current_byte; uint8_t crc = CRC8_INIT; uint8_t crc_bit; /* calculates 8-Bit checksum with given polynomial */ for (current_byte = 0; current_byte < count; ++current_byte) { crc ^= (data[current_byte]); for (crc_bit = 8; crc_bit > 0; --crc_bit) { if (crc & 0x80) crc = (crc << 1) ^ CRC8_POLYNOMIAL; else crc = (crc << 1); } } return crc; }

Width 8 bit

Protected Data read and/or write data

Polynomial 0x31 (x8 + x5 + x4 + 1)

Initialization 0xFF

Reflect input False

Reflect output False

Final XOR 0x00

Examples CRC (0xBEEF) = 0x92

Table 30 I2C CRC properties.


4 Mechanical specifications

4.1 Package Outline

Figure 3 schematically displays the package outline. The notched corner of the protection membrane serves as a polarity mark to indicate pin 1 location. Nominal dimensions and tolerances are listed in Table 31.

Figure 3: Packaging outline drawing of the SCD4x: (left) top view and (right) side view. Nominal dimensions and tolerances are listed in millimeters.

Dimension a b c d e f

Nominal [mm] 10.1 10.1 8.5 7.8 5.5 0.8

Tolerance [mm] ± 0.3 ± 0.3 ± 0.2 ± 0.2 ± 0.3 ± 0.2

Table 31: Nominal dimensions and tolerances SCD4x (all in mm). The weight of the sensor is approx. 0.6 g.

Note that the white protection membrane on top of the sensor must not be removed or tampered with to ensure proper sensor operation.

4.2 Land Pattern

Recommended land pattern, solder paste and solder mask are shown in Figure 4.

Figure 4: SCD4x Footprint (top view): landing pads (a), solder mask (b) and solder paste (c).


4.3 Tape & Reel Package

Figure 5: Technical drawing of the packaging tape with sensor orientation in tape. Header tape is to the right and trailer tape to the left on this drawing. Dimensions are given in millimeters.

4.4 Moisture Sensitivity Level

Sensirion SCD4x sensors shall be treated according to Moisture Sensitivity Level 3 (MSL3) as described in IPC/JEDEC J-STD-033B1. Exposure to moisture levels or solder reflow temperatures, which exceed the limits as stated in this document, can result in yield loss and reliability degradation. The manufacturing floor time (out of bag) at the customer’s end is 168 hours at normal factory conditions (≤30°C and 60%RH). If sensors are not mounted within this time, or have been exposed to higher temperatures and humidity (>30°C and >60%RH), or there is any doubt about the airtight integrity of the dry pack, the parts should be baked (baking parameters see Table 32). The maximum allowed baking temperature is 40°C if the sensors are inside the reel.

Table 32: Baking condition SCD4x if floor time (168 h) with open bag is exceeded.

SCD4x package type Baking temperature Min. baking time Baking condition

Sensors removed from tape 90 °C 48 hours RH < 5 %

Sensors in tape 40 °C 23 days RH < 5 %


4.5 Soldering Instructions

For soldering, standard reflow soldering ovens may be used. The sensors are designed to withstand soldering profile according to IPC/JEDEC J-STD-020 with a maximum peak temperature of 235°C during up to 30 sec for Pb-free assembly in IR/Convection reflow ovens. Note that due to the comparably large size of the SCD4x sensor significant temperature differences across the sensor element can occur during reflow soldering. Specifically, the temperature within the sensor cap can be higher than the temperature measured at the pad using usual temperature monitoring methods. Care must be taken that a temperature of 235° C is not exceeded at any time in any part of the sensor. Do not apply any board wash process step subsequently to the reflow soldering. Note that the dust cover on top of the cap must not be removed or wetted with any liquid. Finally, the SCD4x is not compatible with vapor phase reflow soldering.

Average ramp-up rate < 3 °C / second

Liquid phase

TL tL

< 220 °C < 60 seconds

Peak temperature

TP tP

= 235 °C < 30 seconds

Ramp-down rate < 4 °C / seconds for temperature > TL

Table 33 Soldering profile parameter

4.6 Traceability

All SCD4x sensors have a distinct electronic serial number for identification and traceability (see chapter 3.9.2). The serial number can be decoded by Sensirion only and allows for tracking through production, calibration, and testing.

5 Ordering Information Use the part names and product numbers shown in the following table when ordering the SCD4x CO2 sensor. For the latest product information and local distributors, visit http://www.sensirion.com/.

Part Name Description Ordering quantity (pcs) Product Number

SCD40-D-R2 SCD40 CO2 sensor SMD component as reel, I2C 600 sensors per reel 3.000.521




SEK-SCD41-Sensor SEK-SCD41-Sensor set; SCD41 on development board with cables

1 3.000.455

SEK-SensorBridge Sensor Bridge to connect SEK-SCD41-Sensor to computer

1 3.000.124

Table 34 SCD4x ordering options

6 Revision History

Date Version Page(s) Changes

January 2021 1 all Initial release

http://www.sensirion.com/


Important Notices

Warning, Personal Injury

Do not use this product as safety or emergency stop devices or in any other application where failure of the product could result

in personal injury. Do not use this product for applications other than its intended and authorized use. Before installing,

handling, using or servicing this product, please consult the data sheet and application notes. Failure to comply with these

instructions could result in death or serious injury.

If the Buyer shall purchase or use SENSIRION products for any unintended or unauthorized application, Buyer shall defend, indemnify and

hold harmless SENSIRION and its officers, employees, subsidiaries, affiliates and distributors against all claims, costs, damages and

expenses, and reasonable attorney fees arising out of, directly or indirectly, any claim of personal injury or death associated with such

unintended or unauthorized use, even if SENSIRION shall be allegedly negligent with respect to the design or the manufacture of the

product.

ESD Precautions

The inherent design of this component causes it to be sensitive to electrostatic discharge (ESD). To prevent ESD-induced damage and/or

degradation, take customary and statutory ESD precautions when handling this product.

See application note “ESD, Latchup and EMC” for more information.

Warranty

SENSIRION warrants solely to the original purchaser of this product for a period of 12 months (one year) from the date of delivery that this

product shall be of the quality, material and workmanship defined in SENSIRION’s published specifications of the product. Within such

period, if proven to be defective, SENSIRION shall repair and/or replace this product, in SENSIRION’s discretion, free of charge to the

Buyer, provided that:

notice in writing describing the defects shall be given to SENSIRION within fourteen (14) days after their appearance;

such defects shall be found, to SENSIRION’s reasonable satisfaction, to have arisen from SENSIRION’s faulty design, material, or workmanship;

the defective product shall be returned to SENSIRION’s factory at the Buyer’s expense; and

the warranty period for any repaired or replaced product shall be limited to the unexpired portion of the original period. This warranty does not apply to any equipment which has not been installed and used within the specifications recommended by

SENSIRION for the intended and proper use of the equipment. EXCEPT FOR THE WARRANTIES EXPRESSLY SET FORTH HEREIN,

SENSIRION MAKES NO WARRANTIES, EITHER EXPRESS OR IMPLIED, WITH RESPECT TO THE PRODUCT. ANY AND ALL

WARRANTIES, INCLUDING WITHOUT LIMITATION, WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR

PURPOSE, ARE EXPRESSLY EXCLUDED AND DECLINED.

SENSIRION is only liable for defects of this product arising under the conditions of operation provided for in the data sheet and proper use

of the goods. SENSIRION explicitly disclaims all warranties, express or implied, for any period during which the goods are operated or

stored not in accordance with the technical specifications.

SENSIRION does not assume any liability arising out of any application or use of any product or circuit and specifically disclaims any and

all liability, including without limitation consequential or incidental damages. All operating parameters, including without limitation

recommended parameters, must be validated for each customer’s applications by customer’s technical experts. Recommended

parameters can and do vary in different applications.

SENSIRION reserves the right, without further notice, (i) to change the product specifications and/or the information in this document and

(ii) to improve reliability, functions and design of this product.

Copyright © 2021, by SENSIRION. CMOSens® is a trademark of Sensirion. All rights reserved

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Sensirion AG Laubisruetistr. 50 CH-8712 Staefa ZH Switzerland phone: +41 44 306 40 00 fax: +41 44 306 40 30 [email protected] www.sensirion.com

Sensirion Inc., USA phone: +1 312 690 5858 [email protected] www.sensirion.com

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To find your local representative, please visit www.sensirion.com/distributors

SCD4x Breaking the size barrier in CO

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